Heart-rate variability (HRV) refers to the phenomenon of variation in the time intervals between normal heart beats (non-arrhythmic heart beats). In a normal healthy state, the time intervals between successive heart beats may vary significantly during rest or in response to a change in activity (e.g. getting up from a sitting position), indicating a healthy and flexible interplay between the autonomic nervous system and the cardiovascular system. Higher values of HRV parameters, such as the standard deviation of the time intervals between successive normal heart beats (the standard deviation of NN intervals or SDNN), and the square root of the mean of the squares of time intervals between successive normal heart beats (the root-mean-square of successive differences or rMSSD), indicate better health and wellbeing. Lower values of HRV parameters may indicate high stress, poor rest or sleep, as well as certain pathological conditions. In sports and fitness, higher HRV before a workout was shown to induce better performance in the workout. In cardiology, patients with lower HRV after a myocardial infarct were shown to have higher incidence of cardiac arrhythmias and increased all-cause mortality rates. The joint taskforce of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology [Eu. Heart Journal. 17, 354-381; 1996 and Circulation Journal 1; 93(5):1043-65] issued a guidelines statement emphasizing the importance of monitoring HRV and outlined the clinical benefits of HRV improvement. Some research reports [Circulation Journal. 102:1239-1244, 2000] showed that HRV measurements from a 2 minute rhythm strip is sufficient for determining increased risk of cardiovascular morbidity and mortality. Others have reported [Journal of Cardiovascular Electrophysiology 25(7):719-24; 2014] the close relations between HRV and cardiac arrhythmias such as atrial fibrillation (AF) and emphasize the importance of monitoring HRV for preventing cardiac arrhythmias.
Neuromodulation, that is to say, nerve stimulation, may be used to treat various conditions, ranging from psychiatric disorders, such as depression, to physiological conditions, such as chronic pain. In particular, neuromodulation of peripheral nerves may be used to modulate the autonomic nervous system. For example, neuromodulation of the median and ulnar nerves in the forearm may be used to enhance parasympathetic activity of the vagus nerve and depress sympathetic activity of the cardiac nerve, and thereby to reduce heart-rate, regulate heart rhythm, and correct and prevent cardiac arrhythmias. Neuromodulation of the median and ulnar nerves may also be used to induce a neurohormonal response: secretion of endorphins, decrease of stress related hormones, and thereby to reduce stress and improve sleep.
Portable devices for monitoring and treating various physiological conditions allow for real-time treatment, whether in response to an acute crisis, or to effect a general state of wellness and improved mood. Recent years have seen a sharp rise in the number of consumer wearable devices designed to monitor physiological conditions, from devices designed to track vital signs, such as heart-rate or blood pressure, to recreational wearable devices, such as wearable fitness trackers. Many of these devices are aimed at monitoring and are not designed to provide treatment. Specifically, many wearable devices on the market, whether medical or recreational, for monitoring heart activity and function (e.g. heart-rate, HRV) do not offer treatment.